For myself, the appeal of a great big switch that I can smack immediately without having to hunt for it is a good thing. I have had to use e stop switches in emergency situations with power carts on the flight line. Your brain doesn't work clearly enough under a panic situation to flip four or five switches to kill power, just go for the red button!

We had a power cart that liked to spike the voltage way over its rating, causing electrical components to release the smoke!

Sure, that's what I included in my rendition of the drawing and that P-J and others have done in a different way by shunting current to ground to trip the GFCI.

In hindsight though what thargrav suggests is more like a mechanical master disconnect, someplace you can see it or at least with a lockout capability. Certainly unplugging the control system would fit this as long as you can observe that someone hasn't done you a "favor" and plugged it back in.